This invention relates to methods of making certain functionalized oils having conjugated aliphatic double bonds; adhesive compositions containing those functionalized oils, and aqueous polyurethane dispersions prepared using those functionalized oils.
Various types of plant oils are known as “drying oils” because they have the ability to polymerize in the presence of oxygen to form a hard coating. Examples of such drying oils are linseed oil and tung oil. These oils are mainly glycerides of unsaturated fatty acids. The number and location of carbon-carbon double bonds in the fatty acids determines the extent to which the oil will polymerize and dry. In general, increasing the content of double bonds in the fatty acid groups improves the drying ability. Conjugated double bonds are usually more reactive than nonconjugated double bonds. Tung oil is an especially valuable drying oil. It contains a high proportion of fatty acid groups having three conjugated double bonds. Conjugated double bond-containing oils polymerize well in the presence of oxygen at room temperature to form a high quality, highly water-resistant coating.
Attempts have been made to develop synthetic polymer systems that have drying properties similar to those of the “drying” plant oils. One method of accomplishing this is to incorporate the double bond structure from these drying oils into a synthetic polymer network. To do this, it is necessary to introduce functional groups onto the oils.
A convenient way of introducing functional groups such as hydroxyl groups onto a drying oil is to conduct a transesterification reaction between the oil and a polyol compound. This reaction creates a mixture of (mainly) mono- and di-fatty acid esters having one or more free hydroxyl groups. The free hydroxyl groups are reactive species that can be used to bond the functionalized oil into a variety of polymer structures such as alkyl resins and polyurethanes.
The transesterification reaction to form a functionalized oil does not work well using conventional polyols when the starting oil has highly reactive unsaturation, as is present in tung oil, for example. The usual polyols of choice, glycerine and pentaerythritol, require temperatures of 240° C. or higher in order to complete the transesterification reaction in reasonable lengths of time. The more reactive plant oils react under the conditions of the transesterification, increasing in viscosity and discoloring so badly that the product is not useful. Often, an intractable mass is created. There is also the risk of fire or explosion due to the exothermic auto-polymerization of the oil that can occur at those reaction conditions.
As a result, functionalized plant oils having significant proportions of highly reactive double bonds, such as conjugated double bonds, have not been successfully prepared on a commercial basis.
It would be desirable to provide a process by which highly reactive plant oils, particularly those containing a significant proportion of fatty acids having conjugated double bonds, can be functionalized to form good quality, low color and low viscosity products.